To answer your questions:
Yes, you can have a html (static) and a blog on the same domain and/or hosting packaged.
You can just install the blog in a folder inside your static website (www.domain.com/blog) or as a subdomain (blog.domain.com) and just link the blog inside your static website.
I used to have all static websites and in time changed then over to WordPress. I had to learn this, but if you are a designer and can read, you should be able to handle the WP within 2 months fully, including making adjustments to the php files and plugins.
For the 404 I do not know, but a simple search in this forum will give you an answer.